1. Field of the Invention
This invention relates to doctor blade systems for applying coatings in a printing or coating process, and in particular to a new design for the doctor blade head.
2. Description of Related Art
In the application of liquid substances to a moving web or successive sheets of material, it is considered well known in the art to apply the liquid using a rotating transfer roller, and to directly apply the liquid uniformly onto the roller by means of a doctor blade assembly. The doctor blade assembly generally includes a reservoir chamber extending the length of the transfer roller and in contact with the circumferential surface thereof, and a pair of doctor blades extending longitudinally on either side of the chamber. The doctor blades are angled obliquely toward the transfer roller surface, and serve both to seal the reservoir chamber to the roller and to form a uniform film of liquid on the roller transfer surface. The assembly also must include some means to seal the reservoir chamber at the ends of the roller, so that the liquid is not flung from the roller into the surroundings, and so that the liquid may be pumped through the reservoir during the transfer process. Such transfer systems are used in flexographic and gravure printing, adhesive applicators for substrates such as paper or plastic, coating applicators in many different industrial processes, and the like. Exemplary system are described in U.S. Pat. Nos. 4,821,672 and 6,576,059 issued to Nick Bruno.
It is apparent that the doctor blade head must provide uniform coating of the transfer roller to the utmost extent, so that the printed output is as perfect as possible. Factors that may cause defects in the liquid layer on the transfer roller may include the transfer roller itself, which is furnished with a micro-etched pattern designed to sustain the liquid film that is transferred to the printing or coating roller. The pattern may also carry air into the doctor blade cavity and cause bubbles to form in the coating liquid in the cavity, leading to defects in the coating and printing drop-outs in the final product. This effect is also exacerbated by the rotational velocity of the transfer roller and printing roller, and may limit the production speed of the printing press.
Indeed, the doctor blade cavity may be viewed as a closed space having fixed side and end wall, except for the rapidly and constantly moving side wall formed by the transfer roller engaged by the doctor blade head. In the prior art the cavity is typically a flattened rectangular chamber, and the fluid flow is end-to-end through the cavity. It is quite possible for turbulence to occur within the flowing liquid, which retards the flow rate and requires higher pumping pressure to maintain the fluid flow through the chamber. Turbulence may be increased by the motion of the transfer roller surface forming one side of the cavity, again limiting the speed of the printing press.
In addition to the issue of turbulence, the pump that provides the pressurized fluid to the cavity typically creates pulses of pressure, particularly since pneumatically operated piston pumps are easiest to use and maintain in a transfer coating machine. Instability in the fluid pressure may also contribute to turbulence in the fluid and an ultimate degradation in printing quality.